This invention relates to a method and apparatus for removing volatile contaminants from the phreatic zone, also known as the zone of saturation, or, ground water.
Contamination of the phreatic zone with potentially hazardous materials is a common problem facing industry, the government and the general public. Frequently, as a result of spills, leakage from storage facilities or surface discharges, contaminants percolate into ground water, thereby posing a health threat to drinking water supplies. While groundwater is not as susceptible to pollution as surface water, once polluted, its restoration is difficult and long term. Various methods for withdrawing and treating contaminated groundwater have met with limited success. Typically, groundwater is removed from the phreatic zone, treated and then returned to the phreatic zone. This method involves great expense. A need exists for a method and apparatus for removing volatile contaminants from the phreatic zone without the expense of removing and returning the groundwater to the phreatic zone.
One previous attempt at solving these problems involved a closed-loop device which included one or more contaminant withdrawal wells surrounded by multiple air reinjection wells connected by a conduit. This device utilized the principle of sparging. Sparging is the process of forcing air through water to remove undesirable volatile organic contaminants (VOC's). The above described device formed air channels within the phreatic zone that were distributed in three-dimensions. This approach may lead to uncontrolled distribution of contaminants.
The method and apparatus of the present invention also utilizes the principle of sparging. The present invention is advantageous in that it provides an artificially permeable environment that permits sparging to occur. The method involves digging a trench into the phreatic zone downstream of the contaminated water flow. A horizontal injection pipe is preferably laid at the bottom of the trench and the trench is then filled with gravel just above the level of the water table. The injection pipe may be oriented either horizontally or vertically within the trench. An extraction pipe is then laid on the gravel and the trench is filled up to the ground surface preferably with more gravel. The extraction pipe is preferably oriented horizontally. Lastly, a seal may be formed over the trench, preferably at the ground surface to prevent contaminants from escaping to the atmosphere.
When the present invention is operating, air is forced into the injection pipe and out into the groundwater. Volatile contaminants dissolved in the groundwater then evaporate into the injected air. The air/contaminant gas mixture bubbles up through the trench and above the water table where the extraction pipe removes it for treatment. The perpendicular flow of the groundwater allows the trench to remove the volatile contaminants from the groundwater as it moves through the trench. Practically, there is no removal of the groundwater from the phreatic zone.
It may be appreciated that the method and apparatus may be used as a closed loop. The air withdrawn from the extraction pipe may be treated on the surface and recycled back into the injection pipe where the air may once again carry contaminants.
In addition, ammonium ortho phosphate may be trickled into the trench to biodegrade the biodegradeable contaminants.
Objects and advantages of the present invention will be readily apparent upon a reading of the following description.